Sheep shearing is a demanding occupation that depends as much upon the skill of the individual shearer as it does on the tools he uses to be successful. Like the skills of the individual shearers, the designs of shearing devices vary widely; however, the commercially available shearing devices sold by most manufacturers and used by the vast majority of shearers tend to have a common design that dates back to the turn of the century. In this common design, an elongated fork member is mounted within a hollow handpiece for pivotal movement about an axis. The handpiece has a comb member rigidly mounted to its front and the elongated fork has a cutter member attached to one of its ends. On a portion of the comb, there is a planar face and the cutter is supported to slidingly engage it. In operation, the fork is pivotally moved about its axis which, in turn, reciprocally moves the cutter along an arcuate path across the planar face of the comb. To adjust the pressure with which the cutter bears against the comb, an arrangement is provided that includes a pin or similar member which abuts the fork at a location between the fork's pivotal axis and the cutter which is attached to one of the ends of the fork.
Typical examples of this basic design are illustrated in U.S. Pat. No. 3,467,204 to Jenkinson (see his FIG. 1) and U.S. Pat. No. 4,094,065 to Geary (see his FIG. 1). Variations in this common design are illustrated in U.S. Pat. Nos. 814,113 to Burley, 1,723,323 to Bartlett, 2,080,451 to Wilcox, 2,081,318 to Wright, and 2,232,361 to Bartlett; however, all of these shearing designs have an inherent design defect. Specifically, the arrangements for adjusting the pressure with which the cutter bears against the comb include members which provide a forward thrust. That is, the adjustable force applied to the fork and cutter is not applied symmetrically about an axis that is perpendicular to the planar face of the comb. Rather, it is applied at an angle to this plane as best seen in FIG. 1 of the patents to Jenkinson and Geary. As explained by Burley in his lines 17-19 of his column 1, this forward thrust is provided in order to permit the wear of the cutter to be taken up. Unfortunately, there are several inherent drawbacks in using this forward thrust. First, it applies a force to the pivot bearing of the fork in a direction which is not aligned with the pivotal axis of the fork. This causes irregular wear in the bearing and eventually leads to excessive play in the movement of the fork. Second, and perhaps of more significance, adjustments in the amount of forward thrust provided by the pin member are achieved by physically moving the pin member forward or rearward with the result that the pivotal axis of the pin member shifts and is different for each force setting. Because of this shifting of the pivotal axis of the pin member, the pivotal axis of the fork (which is fixed) and the pivotal axis of the pin member are, at best, only colinear at one force setting. Consequently, at all but this one force setting, the radius of curvature of the fork's movement in a plane parallel to the planar face of the comb has a different center than that of the pin member in this same plane. The result is a non-uniform application of force to the cutter as it reciprocally moves along its arcuate path across the planar face of the comb.
In most of these prior art shears, the pivotal axis of the pin member is forward of the pivotal axis of the fork so that the cutter bears against the comb with more pressure in the middle of its arcuate path across the comb than at the ends. In practice, the shearer compensates for this non-uniform application of pressure by adjusting the pin member to be tight enough to apply the desired pressure at the ends of the cutter's arcuate path so that it will cut along the full length of the path; but, the effect of this is that too much pressure is applied in the middle of the path and the planar face of the comb is literally eaten away in this area. Unless the planar face of the comb is again ground flat, the fork and cutter begin to rock about the longitudinal axis of the fork reducing the cutting efficiency of the shearing device and significantly reducing the quality of the cut. Further, this rocking creates excessive vibrations in the shearing device adding to the strain put on the shearer and greatly reducing his efficiency and control of the shears. Also, the need for frequent grinding of the comb's planar face significantly reduces the comb's life and greatly increases the cost of operating the shears.
With the shears of the present invention, these problems are overcome because the tensioning arm or pin member of the present invention always has the same pivotal axis and the axis is colinear with the pivotal axis of the fork. Further, the force applied to the tensioning arm is applied symmetrically about and axially along this common pivotal axis with no forward thrust created at all; and, the force applied by the tensioning arm to the fork and cutter is applied symmetrically about and axially along an axis which is perpendicular to the planar face of the comb member. Since there is no forward thrust component in these forces and since the pivotal axis of the tensioning arm is fixed and colinear with the axis of the fork, the cutter of the present shears applies an even pressure to the comb along its entire path of travel across it resulting in a cool and smooth running shearing device which provides superior performance and requires less maintenance, particularly to the pivot bearing of the fork and the planar face of the comb.
Examples of other shearing devices in the general area are U.S. Pat. Nos. 5,166 to VanDoren, 155,855 to Burgess, 547,718 to Fletcher, 926,727 to Cahill (which discloses no means at all to adjust the pressure between the cutter and comb other than by raising or lowering shaft 9 to move the pivotal axis of the fork; but, this is totally ineffective because this will merely rock the back, driven end of the fork up and down and off-set the cutter on the comb), U.S. Pat. Nos. 1,227,572 to Bodene, 1,567,110 to Bristow, 1,644,141 to McArdle, and 1,646,470 to Wright.